Although many silicate compounds exist, only the alkali-metal silicates are capable of being dissolved in water and of these the best known are sodium silicate and potassium silicate. Sodium silicate is a generic term for a family of chemical compounds composed of sodium oxide (Na.sub.2 0) silica (SiO.sub.2) and water.
Aqueous solutions of sodium silicate and potassium silicate are commonly used as adhesives and binders. The usefulness of such adhesives is directly dependent on its bonding capacity and the compatability with adsorbing material with which it is employed. A major problem of known sodium silicate solutions has been its limited bonding strength when used with material or objects which are subject to stress and vibration.
Aqueous sodium silicate solutions or potassium silicate solutions having different alkalinities and concentrations may be prepared by adjusting the proportion of silica (SiO.sub.2) to sodium oxide (Na.sub.2 0) or potassium oxide (K.sub.2 0) and by controlling the percentage of total solids in the solution. Using sodium silicate as an example, the proportion of SiO.sub.2 to Na.sub.2 O is commonly expressed on a ratio basis; typical formulations range from Na.sub.2 O. 3.9SiO.sub.2, having a SiO.sub.2 /Na.sub.2 O ratio of 3.9, to Na.sub.2 O. 1.58SiO.sub.2, having a SiO.sub.2 /Na.sub.2 O ratio of 1.58.
The viscosity and total solids contents of aqueous sodium silicate solutions are functions of the SiO.sub.2 /Na.sub.2 O ratio: an aqueous sodium silicate solution having a SiO.sub.2 /Na.sub.2 O ratio of about 2.00 has a minimum viscosity and a maximum total solids content. If the total solids content is held constant and the SiO.sub.2 /Na.sub.2 O ratio is other than 2.00, the aqueous sodium silicate solution will have an increased viscosity. Similarly if the viscosity is held constant and the SiO.sub.2 /Na.sub.2 O ratio is other than about 2.00, the aqueous sodium silicate solution will have a decreased total solids content.
Sodium silicate solutions having smaller SiO.sub.2 /Na.sub.2 O ratios of approximately 1.87 to 2.40 are sticky and tacky but change slowly from liquid to solid form with a relatively large loss of water. Solutions having larger SiO.sub.2 /Na.sub.2 O ratio in the 2.8 to 3.8 range change from liquid to semi-solid form quickly with a relatively small loss of water. These are valuable as adhesives. The characteristic ability of aqueous sodium silicate solutions having large SiO.sub.2 /Na.sub.2 O ratios to set quickly on initial application and to form a strong durable bond with a minimum of penetration into the joined matter is the principle reason for its wide use as an adhesive.